The present invention relates to the field of imaging samples with radiation in the infra-red (IR) and Terahertz frequency range. More specifically, the present invention relates to apparatus and methods for imaging samples in three dimensions using electromagnetic radiation in the higher Gigahertz (GHz) and the Terahertz (THz) frequency ranges. However, in this type of imaging technology, all such radiation is colloquially referred to as THz radiation, particularly that in the range from 25 GHz to 100 THz, more particularly that in the range of 50 GHz to 84 THz, especially that in the range from 100 GHz to 50 THz.
Recently, there has been much interest in using THz radiation to look at a wide variety of samples using a range of methods. THz radiation has been used for both imaging samples and obtaining spectra. Work by Mittleman et al, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 2, No. 3, September 1996, page 679 to 692 illustrates the use of using THz radiation to image various objects such as a flame, a leaf, a moulded piece of plastic and semiconductors.
THz radiation penetrates most dry, non metallic and non polar objects like plastics, paper, cardboard and non polar organic substances. Therefore, THz radiation can be used instead of X-rays to look inside boxes, cases etc. THz has lower energy, non-ionising photons than X-rays, hence, the health risks of using THz radiation are expected to be vastly reduced compared to those using conventional X-rays.
There is considerable interest in both medical and non-medical fields in the production of 3D images. For example, in dentistry the ability to produce 3D images of a tooth would enable dentists to locate exactly where caries (tooth erosion) or other abnormalities occur in the tooth. Most of the conventional imaging modalitiesxe2x80x94X-Ray, MRI, etc.xe2x80x94are handicapped by the fact that they can intrinsically only produce 2D images, with 3D images possible only by translating the patient or body part through the X-Ray beam or through the magnetic field in the case of MRI.
The use of THz for imaging the internal structure of a flat object (a floppy disc) has been described in EP 0 864 857. Here, the inventors measured reflection of a beam of THz radiation to produce an image of the internal structure of the sample.
However, this method is not suitable for obtaining 3D images of objects where the front and back surfaces are curved. Most objects have interfaces and/or external surfaces which are non-planar, i.e. have substantial radii of curvature. If a beam is reflected from a curved surface, it is reflected at an angle to the incident beam. The method of EP 0 864 857 does not show how to obtain an image when the radiation is reflected from a curved surface.
Also, partially absorbing objects give rise to weak reflections from buried layers resulting in long absorption lengths for certain reflected pulses. This limits the thickness of objects which can be accurately imaged in 3D using THz reflection data alone.
The present invention addresses the above problems in a first aspect provides a method of imaging a sample, the method comprising the steps of:
(a) irradiating the sample to be imaged with an irradiating beam of pulsed electro magnetic radiation with a plurality of frequencies in the range from 25 GHz to 100 THz,
(b) detecting both the radiation transmitted through the sample and the radiation reflected by the sample;
(c) generating an image of the sample from the radiation detected in step (b).
Collecting both the reflected and transmitted radiation allows a greater range of curved surfaces to be measured. Hence, the method of the present invention is capable of imaging a sample of virtually any shape. The collection of both the transmitted and reflected radiation also allows a compositional image of the sample to be obtained.
Radiation transmitted through the sample is primarily used to determine the sample shape and the composition. Radiation which is reflected from the sample is primarily used to measure the positions of dielectric surfaces within the sample in addition to giving shape information. This technique allows the curvature of both internal and external surfaces to be measured. Thus, using both reflected and transmitted radiation is an extremely powerful tool to determine the three dimensional compositional structure of the object.
Taking a uniform sphere as a simplified example. In such an example there are no internal interfaces. Therefore, the pulse is at either transmitted through the sphere or will be reflected on entering or exiting the sphere. Subdividing the sample into a 2D array of pixels and measuring the time of flight of the transmitted pulse through the sample will allow the thickness of the sample to be determined at each pixel. However, this will not determine the shape of the sample as the position and shape of the front interface is not known. The shape of the front interface can be determined from the time of flight of the pulse which is reflected on entering the sphere. Thus, it is possible to obtain information about the shape of a sample by plotting the difference between the time of flight of the transmitted and reflected pulses relative to the time of flight of the reflected pulse.
Therefore, the step of generating the image preferably comprises the steps of calculating the time of flight of the pulse transmitted through the sample; calculating the time of flight of a pulse reflected from an interface or surface of the sample; and plotting the difference or a function of the difference of the time of flight of the transmitted pulse and the reflected pulse relative to the time of flight of the reflected pulse.
A function of the difference can be plotted in order to correct for variations in the refractive index of the sample.
In the case of a sphere, theoretically, the pulse reflected on exiting the sphere could be used to determine the shape of the sample in conjunction with the pulse reflected on entering the sphere. However, it is not desirable to use the pulse reflected on exiting the sample as it will be scattered through a fairly large angle, possibly outside the range of the detector. Further, as the reflected pulse has passed twice through the sample, it is likely to be very weak.
The present invention can be used to image far more complex objects than the above uniform sphere. As mentioned above, it is difficult to detect a reflected pulse from the interfaces which are deepest within the sample. Such a complex sample is measured using a reflected radiation detector (or detectors) which is located at the same side on the sample as the incident THz pulse and a transmitted radiation detector (or detectors) which is located on substantially the opposite side of the sample to the incident THz pulse. In a sample with many interfaces, some of the radiation detected by the transmitted radiation detector will have been transmitted through the whole of the sample. However, some of the radiation collected will have undergone multiple reflections. For example, radiation can be reflected back into the sphere from the sphere""s external surface onto an internal interface. The pulse is then reflected for a second time at the internal interface out of the sphere. This reflected pulse will be collected by the transmitted radiation detector. The position of interfaces deep within the sample can be determined by looking at the signal due to such doubly reflected pulses or pulses which have undergone an even number of reflections.
Therefore, preferably, the step of generating the image comprises the step of extracting the parts of the detected transmitted pulse which are due to an even number of reflections within the sample, and determining the position of an interface using the said signal caused by said even number of reflections.
In order to be able to directly compare the reflected and transmitted signals, it is preferable if a reference signal is provided. Said reference signal is preferably provided by a reflection off an object which is a known distance with respect to either the source of THz radiation or the sample being imaged. The reflection may be taken from an object, which is preferably planar located between the source of the radiation and the sample being imaged and is preferably taken from a reflection off a component of the source itself.
In addition to collecting both transmission and reflection data, it is preferable if the resolution of the system is not limited by the diffraction limit. Therefore, it is preferable if the beam which irradiates the sample has a beam diameter which is smaller than the smallest wavelength of radiation in the pulse of electromagnetic radiation.
To obtain an image of the whole sample, the sample is preferably subdivided into a 2 dimensional pixel array. The radiation which is either reflected by or transmitted through each pixel is detected. The image is then generated pixel by pixel.
Preferably, the sample which is to be imaged is placed on a motorised stage, which can be stepped in the both the x and y directions. An image of the entire sample can then be generated pixel by pixel.
Due to the beam diameter being smaller than the wavelength of the radiation, the present invention utilises near-field techniques. Hence, the spatial resolution is not determined by the focused spot size of the THz beam.
The beam of pulsed radiation which is used to irradiate the sample is preferably generated by an emitter which has non-linear optical properties. The material of the emitter is preferably chosen such that when the emitter is irradiated with radiation with a predetermined input frequency or frequencies, the emitter emits a beam with the desired output frequency or frequencies i.e. a frequency or frequencies in the range from 50 GHz to 84 THz. The frequency of the emitted beam is determined by both the frequency or frequencies of the input radiation and the non-linear properties of the emitter itself.
The emitter can be a semiconductor crystal with non-linear optical properties type which allow visible pulses of light (i.e. pulses with wavelengths in the range from 0.3 xcexcm to 1.5 xcexcm) to be converted to pulses with a wavelength in the range from 50 GHz to 84 THz. The emitter may be chosen from a wide range of materials, for example, LilO3, NH4H2PO4, ADP, KH2PO4, KH2ASO4, Quartz, AlPO4, ZnO, CdS, GaP, GaAs, BaTiO3, LiTaO3, LiNbO3, Te, Se, ZnTe, ZnSe, Ba2NaNb5O15, AgAsS3, proustite, CdSe, CdGeAs2, AgGaSe2, AgSbS3, ZnS, DAST (4-N-methylstilbazolium) or Si. Other types of emitter could be used, for example, photoconductive antennas which emit radiation in the desired frequency range in response to irradiation by an input beam having a different frequency and upon the application of a bias to the antenna.
In the case of an emitter which has non-linear optical properties, to keep the input beam in phase with the emitted beam (THz beam), the emitter preferably comprises phase matching means. The phase matching means can be of the type for enhancing the phase matching between at least two different frequency signals propagating in the emitter in response to illumination by at least one incident beam of radiation, the phase matching means having a spatial rotation in its refractive index along a component of the incident radiation beam.
Preferably, the diameter of the beam which irradiates the sample is determined by the diameter of the visible or near-infrared beam which irradiates the emitter. In this situation, there is no need to have extra active optical components between the sample and the emitter to focus the beam. However, in such an arrangement, the sample needs to be positioned close to the emitter. The sample may be mounted directly onto the emitter. Alternatively, the sample may be mounted in very close proximity to the semiconductor emitter. For example, between 10 and 500 xcexcm. Also, the sample may be mounted on a passive optical component which is invisible to THz radiation i.e. a window. The window does not serve to focus the beam.
It may be preferable to separate the emitter and the sample, if the emitter comprises a toxic material, for example, ZnTe.
In the method of the present invention, both transmitted and reflected radiation pulses are measured. When an emitter of the type described above is used, the reflected THz passes must pass back through the emitter (without significant losses) before they are collected as reflected THz for analysis. Therefore, preferably, the emitter is transparent to THz radiation or at least to the radiation of the irradiating beam. Semiconductors with a low carrier doping concentration are useful for this aspect.
The present invention uses both transmission and reflection in order to determine the internal and external shape of the sample. The need to measure, at the reflected signal detector, the pulse which has been reflected once from the curved interfaces located deep within the sample is avoided. However, the reflected signal will be measured from the curved interfaces which are close to the front interfaces. In order to permit a range of sample sizes and radii of surface or interface curvatures which are close to the front of the sample to be measured, the emitter must also be sufficiently large to allow all of the reflected beams to pass back through the emitter. If the emitter is too small, or if the imaging takes place too close to the edge of the emitter, some of the reflections may be blocked by the mount of the emitter. To allow a smaller crystal to be used, it is preferable if just the sample moves in order to image the area of the sample. As the sample moves relative to both the emitter and the input beam of the emitter, a smaller emitter can be used.
To further reduce the size of the emitter, the emitter may be mounted on a xe2x80x9cTHz windowxe2x80x9d. The window material could be for example polyethylene, polythene, high-resistivity silicon, Z-quartz or TPX (poly-4-methylpentene-1), it must be at least substantially transparent to the irradiating beam. The window would preferably be thin, for example, between 50 and 300 microns. This is to ensure that the THz beam diameter is still smaller than the shortest wavelength component when the THz beam reaches the sample. The size of the window is large enough to allow all of the reflected beams to be collected with negligible loss. As the emitter is provided on the window which is substantially transparent to THz, the THz can pass through the mount for the emitter.
Also, using conventional coherent THz detection methods, for example, electro-optic sampling and photoconductive detection, the THz beam must be focused to a point and thus information is lost about the path the different THz beams take following reflection. This problem can be addressed by using a CCD camera. Therefore, it is preferable if a CCD camera is used in the detection the reflected THz beams. This detection method allows reflection techniques to map out the shape of curved surfaces, also, it would be possible to map out differences in shape between internal and external dielectric surfaces.
It should be noted that the CCD camera would probably not be used to detect the THz directly, instead the THz would be converted to a visible or near IR radiation an electro-optical component, the near IR visible radiation would then be collected by the CCD. Preferably, this conversion to IR or visible radiation would be achieved by passing a polarised reference beam with the THz beam through a material which supports the AC pockets effect. The light emitted by the material is then passed through a polariser to the CCD. Only light which has had its polarisation rotated by the THz signal will be transmitted be the polariser into CCD.
Also, when collecting the output light using an off axis parabolic mirror, there is a slight time delay due to the different optical path lengths between the centre and the edge of the mirror. Consequently, the different path lengths of the reflected beams cause the pulses to arrive at different times at the detector. This causes a problem, because it is not easy (if at all possible) to distinguish between a time-shift of a pulse due to the position of an internal dielectric layer and a time shift which is a combination between a reflection from the sample and a different path length due to one of the optical mirrors. This problem can also be addressed by the use of a CCD camera as a detector. A CCD camera can be used to image a 2D region containing all the reflected THz beams, both the temporal and spatial shift if the THz can be measured. In other words, more exact information about the sample can be gained by using a CCD camera.
The CCD technique can be used to collect radiation which is both transmitted and reflected from the sample. As in many situations, the transmitted beam may also be transmitted off axis.
In the method of the present invention, data can be derived by using the time-of-flight method. As the enters the sample, its velocity changes due to variations in the refractive index of the sample. Thus, by measuring the time of flight of the pulse through the sample, an image of the sample shape can be obtained using transmission.
Using the frequency domain analysis techniques of UK application no 9940166.7, the composition of the structure can be determined. In this application, a single frequency from the plurality of transmitted or reflected plurality of frequencies is used to generate the image. In some cases, a narrow range of frequencies or a selection of specific frequencies or frequency ranges is studied. A selected frequency range is taken to be a frequency range typically less than a third of the total range of the passed electromagnetic radiation used to irradiate the sample. More preferably, the selected frequency range is less than 10% of the total frequency range of the passed electromagnetic radiation used to irradiate the sample.
For example, water is a strong absorber of THz radiation. There are xe2x80x9cwindowsxe2x80x9d in the water absorption spectra from 50 GHz to 500 GHz, from 30 THz to 45 THz and from 57 THz to 84 THz. If the sample is irradiated with a range of frequencies from 50 GHz to 84 THz, it may be preferable to generate the image using one or more of the following selected frequency ranges: 50 GHz to 500 GHz, 30 THz to 45 THz and 57 THz to 84 THz. The image may be generated by integrating over the selected frequency range.
Thus, by analysing the transmitted information as above, an image can be created by a single frequency or a selected frequency range. Also, a plurality of images may be derived from a plurality of frequencies or a single image may be derived from two or more distinct frequencies. This is a very powerful analysis and allows variations in the composition of the material to be determined.
The image or images may be generated in a number of ways. For example, a sequence of images may be generated from a plurality of different frequencies.
In general, the present invention will be performed using imaging apparatus which is configured to detect temporal data at each pixel. Preferably, the data is Fourier transformed to give the complex THz electric field in the frequency domain E (xcfx89).
The image can be obtained in a number of ways from the complex THz electric field E(xcfx89), e.g.:
(i) The power spectrum Psample (xcfx89) of the sample and the power spectrum Pref (xcfx89) of the reference signal may be calculated. The image could then be generated by plotting the difference between the two Power spectrums for a given frequency for each pixel at a selected frequency over integrated over a selected frequency range.
(ii) The power spectrum Psample of the sample and the reference power spectrum Pref may be divided to give the transmittance. The transmittance may then be plotted for each pixel at a selected frequency over integrated over a selected frequency range.
(iii) The frequency dependent absorption coefficient xcex1(xcfx89) may be calculated from the complex electric field E(xcfx89) and plotted for each pixel at a selected frequency over integrated over a selected frequency range.
(iv) The frequency dependent refractive index xcex7(xcfx89) may also be calculated from the complex electric field and plotted for each pixel at a selected frequency over integrated over a selected frequency range.
The detected temporal electric field contains both phase and amplitude information which give a complete description of the complex dielectric constant of the medium in the beam path. The sample to be characterised is inserted into the beam and the shape of the pulses that have propagated through the sample or have been reflected from the sample are compared with the reference temporal profile acquired without the sample. The ratio of the complex electric field E(xcfx89) and the reference signal Eref(xcfx89) is calculated to give the complex response function of the sample, S(xcfx89). In the most simple case, the complex response function is given by:                               S          ⁡                      (            ω            )                          =                                            E              ⁡                              (                ω                )                                                                    E                ref                            ⁡                              (                ω                )                                              ∝                                    exp              ⁡                              (                                                                            i                      ⁢                                              xe2x80x83                                            ⁢                      ω                      ⁢                                              xe2x80x83                                            ⁢                      d                                        c                                    ⁢                                      (                                                                  η                        ⁡                                                  (                          ω                          )                                                                    -                      1                                        )                                                  )                                      ⁢                          exp              ⁡                              (                                                      -                                          α                      ⁡                                              (                        ω                        )                                                                              ⁢                  d                                )                                                                        (        1        )            
where d is the sample thickness, c is the velocity of light in vacuum, xcex7 is the refractive index and xcex1 is the absorption coefficient. The experimental absorption coefficient xcex1(xcfx89) and the refractive index xcex7(xcfx89) may then be easily extracted from the magnitude M(xcfx89) and the phase xcfx86(xcfx89) of S(xcfx89), respectively, according to                               α          ⁡                      (            ω            )                          =                              -                          1              d                                ⁢                      ln            ⁡                          (                              M                ⁡                                  (                  ω                  )                                            )                                                          (        2        )                                          η          ⁡                      (            ω            )                          =                  1          +                                    (                              c                                  ω                  ⁢                                      xe2x80x83                                    ⁢                  d                                            )                        ⁢                          φ              ⁡                              (                ω                )                                                                        (        3        )            
Additional terms may be included in equations (1) to (3) to account for reflections at dielectric interfaces of a sample, thus allowing accurate analysis of multilayered samples.
These parameters are simply related to the complex dielectric function ∈(xcfx89) of the sample
xe2x80x83∈(xcfx89)=(xcex7(xcfx89)+ik(xcfx89))2=(xcex7(xcfx89)+ixcex1(xcfx89)c/2xcfx89)2xe2x80x83xe2x80x83(4)
The data derived as discussed in (i) to (iv) above, may be directly plotted either as a colour or a grey scale image where the colour or shade of grey of each pixel represents a given magnitude.
Instead of a single frequency, a selected frequency range could be chosen and the result and data of (i) to (iv) integrated over that range. The integrated data could then be plotted.
It may also be preferred to subdivide the magnitude of the data process in accordance with any of (i) to (iv) above into various bands. For example, all data below a certain value could be assigned the value 0, all data in the next magnitude range could be assigned the value 1, etc. These ranges may have equal widths in magnitude or they may have different widths. Different widths may be preferable to enhance contrast e.g. to emphasise contrast in regions of the sample where there is little variation in the sample absorption of Thz.
Preferably, the present invention uses two or more frequencies. The data from say two frequencies is processed in accordance with any of (i) to (iv) above. The data is then banded as described for a single frequency above.
The data may be split into two hands, one assigned the value xe2x80x9c0xe2x80x9d and the other xe2x80x9c1xe2x80x9d. The data from both frequencies can then be added together using a rule such as a Boolean algebraic expression e.g. AND, OR, NOT, NAND, XOR, etc.
Of course, the present invention also allows images to be compared from two different frequencies. This may be particularly useful to identify a substance where the absorption to THz changes over a certain frequency range.
Thus, complex images can be produced. This system is particularly useful in the detection of breast cancer where both spatial information and compositional information concerning the 3D structure of the breast can be derived. Also, the present invention can be used to image teeth and bone.
The method of the present invention allows the internal composition, shape and the position of the internal surfaces to be determined. Hence, a three dimensional image of the sample can be produced from the methods of the three aspects of the present invention. In a second aspect, the present invention provides an apparatus comprising:
a) means for irradiating a sample to be imaged with an irradiating beam of pulsed electromagnetic radiation with a plurality of frequencies in the range from 25 GHz to 100 THz;
b) means for detecting radiation which is both transmitted through and reflected from the sample; and
c) means for generating an image of the sample from radiation detected in step (b).
For the reasons described above, it is preferable if the imaging is performed in the near-field regime. Therefore, it is preferable if the means for radiating a sample comprises an emitter for emitting a beam of radiation with a plurality of frequencies in the range from 25 GHz to 100 THz, the emitter having optical non linear properties, such that when the emitter is irradiated with an input beam with a frequency in the visible or near infra-red frequency ranges, a beam is emitted with frequencies in the range from 25 GHz to 100 THz. Preferably, the input beam of pulsed radiation has a diameter which is smaller than that of the smallest wavelength of the emitted beam.
To image the sample, the sample should be stepped pixel by pixel in two orthogonal directions. Therefore, it is preferable that the apparatus further comprises a motorised stage configured so that it can be stepped pixel by pixel into two orthogonal directions.
The sample itself can be mounted on the motorised stage. Alternatively, both the sample and the emitter can be mounted on the motorised stage.